This application is related to Japanese patent application No. 2000-311612, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an oscillator and a method for adjusting oscillation characteristics of the oscillator. More specifically, the present invention relates to a small oscillator used in mobile telephones, mobile data terminals, wireless LAN transmitter/receivers, satellite communications terminals, GPS receivers and other types of wireless communication devices operating at a high frequency band, and to a method for adjusting the oscillation characteristics of the oscillator. An oscillator according to this invention is well suited to reducing the size of a module that is a major component of the oscillator used particularly in a high frequency band operating at hundreds of megahertz and higher.
2. Description of Related Art
As terminal devices for high frequency communication systems, such as mobile telephones, have gotten smaller, so have oscillation circuit modules, such as voltage control oscillators (VCO), which are one kind of high frequency components used in such terminal devices.
The characteristics of individual components and variations in wiring pattern dimensions in substrates cannot be ignored in such small oscillation modules for high frequency applications, particularly at frequencies of hundreds of megahertz and higher. It is therefore necessary in practice to adjust each individual module so that the oscillation frequency, for example, is within a specific design range.
One method used for this adjustment as taught in Japanese Unexamined Patent Publication HEI6 (1994)-13807 is to adjust an inductor part of an oscillation circuit by changing the length and/or width of a conductor pattern. This is accomplished by cutting a part of a circuit pattern formed on a component mounting surface of a circuit board by mechanical means, such as a sandblasting machine, or optical means, such as a laser beam. Other methods seeking to achieve an even smaller module by means of a multilayer circuit board provide a part of an inductor (see Japanese Patent No. 2662748) or capacitor (see Japanese Patent No. 2531000) of the oscillation circuit in the circuit board layers.
FIGS. 10 and 11 show an exemplary oscillator in which an inductor part of an oscillator is drawn to a circuit board surface. As shown in FIG. 10, a back ground conductor 22, an internal ground conductor 23 and an inductor conductor 24 of a strip line between the conductors 22 and 23 are formed within a printed circuit board 21, forming a tri-plate structure. A part 24a of the strip line inductor conductor is connected through a through hole 25 to a conductor pad 24b mounted on the substrate surface. The electrical length of the inductor can then be changed by appropriately trimming this surface conductor pad 24b on the substrate surface as shown at notches 26 in FIG. 11, thereby adjusting inductance L to vary oscillator characteristics such as oscillation frequency.
Another adjustment method is illustrated in FIG. 12. As shown in this section view of an oscillator, a capacitor of the oscillator is formed inside a substrate, and one electrode of the capacitor electrode disposed on the substrate surface is trimmed to adjust capacitance as a means of adjusting oscillator characteristics. In this example, an inductor of a resonance circuit is incorporated as a strip line 24 of the tri-plate structure inside a printed circuit board 21 as in the above example, and one end of the inductor is exposed via a through hole 25 on to the substrate surface as a surface electrode 28. That is, a capacitor 27 of a resonance circuit parallel-connected to the inductor is mounted within the circuit board with the surface electrode 28 opposed to the internal ground conductor 23 with the intervention of the dielectric circuit board. The surface electrode 28 is trimmed to adjust the electrode surface area, and thereby adjust capacitance, that is, adjust the oscillator.
In the above-described prior-art examples, while a part of the circuit is formed inside the circuit board so as to reduce device dimensions, both of the methods also expose a part of the internal conductor pattern on the component mounting surface of the circuit board so that the exposed part (part 24b in FIG. 10 and part 28 in FIG. 12) is trimmed, to adjust oscillator module characteristics. This method of internalizing a part of the circuit as a means of reducing size is therefore the same as methods in which parts are not internalized in that a trimming pad occupies a certain amount of area on the component mounting surface.
A metal shield cap covering the component mounting surface is provided for most such modules to protect the mounted components and prevent electromagnetic interference with neighboring parts. This shield cap can be mounted either before or after adjusting the oscillator. When the shield cap is installed after trimming a circuit component on the component mounting surface of the circuit board to adjust the oscillator, the adjustment must be accomplished to anticipate any shift in characteristics resulting from the later addition of the shield cap. If the shield cap is installed before the adjustment, a laser can be used for trimming through a trimming slit or hole provided in the shield cap, and the trimming slit or hole is then sealed with a conductive sealant.
In the former method, trimming must allow for any shift in the oscillator frequency or other module characteristics resulting from the addition of the shield cap. As noted above, however, since variations in characteristics of individual modules are great, the adjustment is not possible with a uniform offset, and the precise adjustment is not possible with this method. In the latter method, dust and debris from the trimmed part become trapped inside the shield cap because trimming is accomplished after the shield cap is mounted. Such dust and debris can easily adhere to surrounding parts, resulting in a possible loss of reliability.
One possible method of resolving this problem is to draw an internal conductor to the back side of a circuit board for trimming. In this case, however, a trimming pad must be provided, and this occupies some area on the back of the circuit board. This makes it difficult to shield the back of a module. A shield cap such as provided on the front surface of the circuit board must therefore be provided. This method is therefore not practical for applications seeking to downsize the module.
Another oscillator adjustment method proposing a solution to this problem is taught in Japanese Unexamined Patent Publication HEI9 (1997)-153737. This method accomplishes laser trimming perpendicular from the back of a circuit board to the inside to adjust electrode area of a multilayer internal capacitor.
FIG. 13 is a section view of a module in this method. In this module a capacitor 27 of a resonance circuit is internalized in a circuit board by stacking electrodes 27a and 27b with a dielectric layer of the circuit board therebetween. As indicated by an arrow 28 in FIG. 14, laser trimming perpendicular to and from the back of the module adjusts the area of the electrodes 27a and 27b of the internal capacitor 27, and thus adjusts the module.
While this method solves the surface area problem described above, an electrode structure of a capacitor must be a multilayer structure comprising at least three or four layers in order to achieve a sufficient capacitance adjustment range. This results in a multilayer circuit board, increasing circuit board thickness and cost.
The inventors of the present invention have been provided another solution to the above-mentioned problems as disclosed in Japanese Unexamined Patent Publication No. 2001-007642 (hereinafter it is referred to as a preceding application). In this method, a part of an inductor element of a resonance circuit is disposed within a dielectric layer of a substrate and the internally disposed inductor element is cut simultaneously with the dielectric layer by laser beam irradiation from the back side of the substrate to change the length and/or width of the inductor element pattern, thereby adjusting the inductor of the oscillation circuit.
The preceding application will be explained with reference to FIG. 15 illustrating its major structure. According to the preceding application, a circuit board including a dielectric layer therein is formed in a four-layered structure of a component mounting conductor pattern surface (layer) 32, an internal ground conductor layer 33, a conductor pattern layer 34 and a back ground conductor layer 35. The internal conductor pattern layer 34 serves as an internal strip line conductor pattern 34 and sandwiched between the internal ground conductor layer 33 and the back ground conductor layer 35 to form an inductor of an oscillator in a tri-plate structure.
The internal conductor pattern layer 34 is electrically connected through a via member 36 to a conductor pattern layer 32 on the substrate surface. The surface conductor pattern layer 32 is connected to another oscillator component 31 mounted on the substrate surface. Further, a metal shield cap 38 is provided on the substrate surface, and the back ground conductor layer 35 is provided with a trimming slit 40. A laser beam 41 is irradiated through the slit 40 to simultaneously cut the dielectric layer and the internal conductor pattern layer to change the electrical length of the inductor, thereby adjusting inductance L, i.e., oscillation characteristics such as oscillation frequency.
The preceding application can achieve downsizing of the module. However, the dielectric layer of glass epoxy and the internal conductor pattern layer of Cu or the like are greatly different in laser beam absorption rate. Glass epoxy and Cu show the absorption rate of several tens % and about 5%, respectively, to a YAG laser beam of 1.06 xcexcm wavelength. Therefore it is difficult to adjust the laser beam to an optimum intensity for both of the materials.
Where the oscillation characteristic needs to be greatly varied to reach the desired characteristic by trimming only one part, the amount of the trimming part increases and the width of the part of the inductor is reduced, which raises the resistance at this point and reduces Q value of the inductance L, resulting in deterioration of module performance such as noise characteristics.
Further, trimming in a great amount also takes time. Accordingly, the above-mentioned adjustment method is not sufficient in view of module performance and reliability.
With consideration for the above problems, an object of the present invention is therefore to provide a compact, high performance, high reliability oscillator (module) having a structure which allows an internal conductor pattern primarily of a resonance circuit inductor to be trimmed directly from the back of a circuit board. A further object of the invention is to provide an adjustment method for the oscillator (module).
To achieve the above objects, the present invention provides an oscillator comprising: a circuit board having a dielectric layer therein; an oscillator circuit comprising its components and an inductor element, the components being mounted on a front surface of the circuit board and at least a part of the inductor element being internally disposed in the dielectric layer; and a back conductor provided on a back surface of the circuit board; wherein the back conductor has two or more slits or pinholes through which a laser beam is passed for partially cutting the dielectric layer together with the internally disposed inductor element at plural sites for adjustment of an oscillation characteristic.
In other words, according to the present invention, the slits or pinholes are provided on the back conductor on the back surface of the circuit board and the laser beam is passed through the slits or pinholes to partially cut (trim) the internally disposed inductor element together with the dielectric layer sandwiched between the back conductor and the internal conductor (a part of the internally disposed inductor element) at plural sites, thereby changing the electrical length of the pattern to adjust the oscillation characteristics to a desired level. Upon changing the electrical length, the inductor element is cut at the plural sites, which prevents the width of the inductor at a certain site from being extremely reduced and thus inhibits decrease in Q value of the inductance L.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.